1. Field of the Invention
The present invention relates to an electronic driving apparatus, and particularly to a driving circuit for light emitting diodes (LEDs).
2. Description of Prior Art
Referring to FIG. 2, in a conventional LED driving circuit, a current limiting resistor 11 is provided between a power supply 10 and an LED array 12 formed by an m×n matrix of light emitting diodes (LEDs) 121. A voltage provided by the power supply 10 is U, a resistance of the current limiting resistor 11 is R, a resistance of each LED 121 is RS, and a current of a main path is I. The voltage-current (VF-IF) characteristic of each LED 121 is shown in FIG. 3, and can be expressed by the following equation:VF=Von+RSIF+(ΔVF/ΔT)(T−25° C.)where Von is a threshold voltage of the LED 121When a temperature of the environment is constant, the above equation can be simplified as follows:VF=Von+RSIF
Thus the electrical characteristic of the LED driving circuit can be expressed as:U−mVon=I(R+RS(m/n))Simplifying the above equation yields:U−VX=I(R+RX)whereVX=m*Von, RX=RS(m/n)
Because of the existence of Von, the current I does not change proportionally with the voltage U. For example, when the voltage U changes to 2U, the current I does not double but instead changes to less than 2I. The LED driving circuit cannot regulate the current I proportionally by linearly changing the voltage U provided to the LED array 12. Therefore it is difficult to precisely control the current I.
Similarly, when a quantity of the LEDs 121 or when a form of the LED array 12 is changed (i.e., VX and/or RX is varied), alterations of the voltage U and the resistor R are required in order to control the current I. However, for the reasons described above, such alterations to precisely control the current I are difficult.
Therefore, it is desirable to provide an improved driving circuit which overcomes the above-described disadvantages of the conventional driving circuit.